Quartz glasses are roughly categorized into two groups in accordance with the raw materials therefor, that is, a natural quartz glass which is produced by steps of pulverizing a natural raw material, purification, and melting, and a synthetic quartz glass which is produced from a liquid silicon compound.
Although a natural quartz glass is inexpensive, the impurity content thereof is generally high, and in addition, the quality thereof is not stable. Accordingly, when a natural quartz glass is used as a raw material for forming heat treatment members for semiconductors, crucibles for pulling semiconductor single crystals, optical members, and the like, a high-purity natural quartz glass that is severely selected has been used; however, the impurity content thereof has still been high.
Accordingly, although a natural quartz has been used as a raw material for quartz for long years, due to variation in impurity, depletion of natural resources, environmental pollution caused by development, and the like, a synthetic quartz has been used in recent years.
Heretofore, since a synthetic quartz powder has been formed by using tetramethoxysilane, tetraethoxysilane, silicon tetrachloride, and the like, the synthetic quartz powder is highly pure but is expensive. Accordingly, when a synthetic quartz glass is formed by using the synthetic quartz powder, the manufacturing cost is increased; hence, the synthetic quartz powder has not been suitably applied to industrial uses.
On the other hand, movement towards higher integration density of semiconductor products has been in progress, and in particular, a high-purity synthetic quartz glass containing impurities at a significantly low content has been in demand for use in forming crucible members for pulling semiconductor single crystals. Due to the demand described above, attempts have been made to produce a high-purity synthetic quartz glass powder at a low cost, and methods using an inexpensive water glass as a raw material have been disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 59-54632, 4-349126, and 11-11929.
However, according to the methods disclosed in the above publications, since a hydrous silica gel is first obtained from a water glass, and silica, which is used as a direct raw material for producing a quartz glass, is then obtained by heating the hydrous silica thus formed for dehydration in order to remove water contained therein, the methods described above are not suitably applied to industrial uses in view of energy cost. In addition, since a small amount of water-soluble impurities contained in an aqueous phase is condensed by heating for dehydration, a washing step or the like must be sufficiently performed, and in this point of view, the methods described above are also not suitably applied to industrial uses. Furthermore, minor heavy metals contained in a synthetic quartz powder produced by the above methods cannot sufficiently be removed, and in particular, titanium cannot sufficiently be removed.
As other methods for producing a synthetic quartz glass, a method comprising hydrolyzing an alkoxysilane for gelation, decarbonizing, and subsequently firing at 1,200° C. or more has been disclosed by S. Sakka in “Treatise on Materials Science and Technology”, No. 22, and a method which comprises forming silica from an alkali silicate and then firing has been disclosed in Japanese Unexamined Patent Application Publication No. 11-11931.
However, the silica described above contains relatively a large number of OH groups (silanol groups), and the number of the silanol groups cannot significantly be reduced by firing. When a quartz glass obtained from the silica containing silanol groups as described above is used as a raw material to produce crucibles for pulling semiconductor single crystals or the like, there have been practical problems in that bubbles are generated or the viscosity is decreased when the quartz glass is melted.
Accordingly, as a method for removing silanol groups from silica particles, a technique using the diffusion of silanol groups by holding silica particles in a dehumidified atmosphere at 1,220° C. for 50 hours has been disclosed in Japanese Unexamined Patent Application Publication No. 8-26741. However, since being performed by heat treatment for a long period of time, this method is not suitably applied to industrial uses, and in addition, there has been a problem in that silica particles are devitrified (crystallized).
In addition, in Japanese Unexamined Patent Application Publication No. 2-289416, a method for removing silanol groups by holding a raw material at 600 to 1,000° C. has been described; however, firing for a long period of time must be performed to sufficiently decrease the content of the silanol groups.
Accordingly, an object of the present invention is to provide a method for dehydrating a hydrous silica gel, which method can be suitably applied to industrial uses in view of energy cost, and to provide a method for producing a high-purity synthetic quartz glass powder, which method can be suitably applied to industrial uses in view of energy cost.
In addition, another object of the present invention is to provide a method for producing a high-purity synthetic quartz powder, in which a high-synthetic quartz powder containing an extremely small amount of titanium can be obtained even when a water glass, which is inexpensive, is used as a raw material.
Furthermore, still another object of the present invention is to provide a method for producing a quartz glass, in which a high-purity quartz glass containing an extremely small amount of silanol groups can be efficiently obtained.